Detector types thermal

In order to be consistent with the earlier chapters of this volume the detector advances are segregated into general detector types (thermal, photoconductor and photovoltaic, photoemissive, charge transfer devices, and heterodyne). The final section of this chapter presents future detector and related technology advances (from the author s point of view) that would greatly enhance our ability to use optical and infrared detectors to solve basic problems of our society. 

Detector, type thermal conductivity thermal conductivity flame... 

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

The components in a mixture separate in the column and exit from the column at different times (retention times). As they exit, the detector registers the event and causes the event to be recorded as a peak on the chromatogram. A wide range of detector types are available and include ultraviolet adsorption, refractive index, thermal conductivity, flame ionization, fluorescence, electrochemical, electron capture, thermal energy analyzer, nitrogen-phosphorus. Other less common detectors include infrared, mass spectrometry, nuclear magnetic resonance, atomic absorption, plasma emission. 

There are different types of detectors, including thermal conductivity and electron capture t5q)es. The most common detector is the flame ionization variety (FID). A hydrogen flame is utilized to combust the column effluents. Thermal conductivity detectors do not degrade the effluents and are not as sensitive as flame ionization. Electron capture detection is especially sensitive to halogenated compounds. 

With the controlled atmosphere heated sample holder, it was a simple matter to connect a thermistor-type thermal conductivity cell to the system and, by means of an external multichannel recorder, record the DRS and the evolved gas detection lEGD) curves simultaneously (17). This modification of the apparatus is shown in Figure 9.4. The cell was connected to a Carle Model 1000 Micro-Detector system by means of metal and rubber tubing. The thermal conductivity cell was enclosed by an aluminum block which was heated to 100 C bv means of a cartridge heater. The block was connected to a preheat chamber, also operated at 100 C, which was used to preheat the helium gas stream before it entered the detector. The output from the detector bridge was led into one channel of a four-channel 0-5 mV Leeds and Northrup multipoint strip-chart potentiometric recorder. The temperature programmer from a Deltatherm III DTA instrument was used to control the temperature rise of the DRS cell. Output from the Beckman Model DK-2A... 

Although not specifically delineated, the volume is also divided into three general sections. The first addresses the full spectrum of infrared detectors and contains a limited coverage of all the material presented in subsequent chapters. It serves as an introduction to the volume and presents to the reader an overall view of the present state of the infrared technology art. It also serves as the mortar between the more in-depth discussions which follow. The midsection. Chapters 3,4, and 5, is a detailed analysis of those detector types which are most widely used today thermal, photoconductive/photovoltaic and photoemissive. 

It is generally desirable to have a detector which detects all substances well. Such a device is impossible to achieve, since each detector is based on the measurement of a particular substance property and different substances have correspondingly differing substance properties. There are, however, detectors which are suitable for very broad substance ranges. Flame ionization detectors and thermal conductivity detectors, for example, are virtually universal. Even with these types, though, different properties... 

Photon detection is generally used from the vacuum ultraviolet to the near infrared and thermal detection from the mid-infrared onwards although there is some overlap. Figure 2 shows detector types for optical spectroscopy and detectors. 

Among the variety of detectors, only the thermal conductivity detector (TCD) and the flame ionization detector (FID) are in broad use in PGC. The flame photometric detector, typically used for measuring trace sulfur containing species, and the photoionization detectoi predominately used in environmental monitoring, also see some usage. The variety of detector types available for PGC tends to be limited because of the requirements for robustness and sensitivity to a variety of stream components. In addition, many PGC detectors are not optimized for use with capillary columns. 

To understand the reason for this difference in detector type, consider the effect on a TCD signal if the flow is completely stopped. The detector cell remains filled with a given concentration of analyte and its thermal conductivity continues to be measured at a constant level. However, for a mass flow rate detector like the FID in which the signal arises from a burning of the sample, a complete stop in the flow rate will cause the delivery of the analyte to the detector to stop and the signal will drop to zero. 

Ultraviolet and Infrared Radiation Monitoring Devices. A variety of instruments are commonly used to measure ultraviolet and infrared radiation.They are classified according to the type of detector used, which is generally one of two types thermal detectors or photoelectric detectors. Thermal detectors are those in which the absorbed radiation is degraded to heat and subsequently converted to an electric signal by changing the electric resistance of a filament. Photoelectric detectors are based on the principle that the absorbed photons eject electrons from a material. Most of these instruments are precalibrated by the manufacturer, but should be routinely checked prior to field use. 

Li, is a common reaction employed in neutron detectors, and these detectors are especially sensitive to slow neutrons because of large thermal neutron cross section of B -°. Another neutron detecting scheme using secondary charged particles to ionize the gas is the fission counter. Here, fission fragments do the ionizing and this detector type is also primarily sensitive to slow neutrons. Most detectors used in reactors and health physics instruments detect slow neutrons by one of the above (or similar) reactions. 

A comparison of the remaining two classes, photonic and thermal detectors, shows that their performance is similar from the point of view of the criteria presented in Table 1.2. The main advantage of photonic detectors is their response speed, and in the case of thermal detectors their room-temperature operation and relatively lower price. This means that each of these detector types can find a share of market where its will dominate. 

Detector, type Voltage or mA thermal conductivity 7.5 V thermal conductivity hydrogen flame thermal conductivity 8V... 

Detector, type flame thermal conductivity flame thermal conductivity thermal conductivity thermal conductivity... 

We mentioned that Herschel s detector was a thermometer - the first thermal detector. Thermal detectors respond to the power falling on the detector and they are still used - though they are more sensitive than Hershel s thermometer. However, another detector type is now very important. Called photon detectors, these respond not to the power falling on the detector but on the rate of arrival of photons - discrete packets of energy. We will discuss photons in Chapter 2, and the two detector types in Chapters 3 and 4. 

Infrared detectors are either thermal detectors or photon detectors. These two work in very different ways. Although the operator may not know, or need to know, which type he is using, when selecting a device for a particular application, or predicting its performance, it is important to understand the difference between the detector types. 

Kruse and Skatrud (1997) Uncooled Infrared Imaging Arrays and Systems by Volume Editors P. W. Kruse and D. D. Skatrud, Academic Press, San Diego, CA, Chestnut Hill, MA. This is a comprehensive review, including a chapter on the history of thermal detectors, and chapters on each of the significant thermal detector types. 

The largest item driving cost is the SNR requirement. Wide selections of detectors are available from a host of companies. There are literally thousands of photomultiplier mbe types, array detectors (line and 2-D array), and semiconductor light sensors and thermal infrared detectors covering a spectral range from 150 nm to more than 40 pm. The wavelength range often determines the choice of detector type. 

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